Precise temperature measurement and management are very important across the whole industry. Temperature measurement and management are executed using a temperature sensor that senses a specific temperature. An example of a temperature sensor is a thermistor which is the cheapest and convenient.
A thermistor is a semiconductor which is formed by mixing oxides of cobalt, copper, manganese, iron, nickel, titanium, etc. to have an appropriate resistivity and an appropriate temperature coefficient and then sintering the mixture of the oxides. The conductivity of a thermistor varies according to a change in the temperature of its surroundings. In contrast with general metal, a thermistor has a negative temperature coefficient (NTC) in which a resistance value decreases as a temperature increases. This thermistor is referred to as an NTC thermistor. The NTC thermistor detects a temperature and is usually used in an apparatus which drives a relay switch to generate an alarm signal.
FIG. 1 is a circuit diagram showing a conventional alarm using a conventional thermistor. Referring to FIG. 1, the conventional alarm includes a sensor part (which is defined by the rectangular dotted line) and an alarm signal part. The sensor part includes a thermistor 10, an amplifier 20, and a transistor 30. The alarm signal part includes a buzzer 50 and a light emitting diode 60. The sensor part and the alarm signal part are connected to each other via a relay switch 40.
In an operation of the conventional alarm, as the resistance of the thermistor 10 changes according to a temperature change, a voltage at the negative input port of the amplifier 20 changes. Accordingly, an output voltage of the amplifier 20 changes. The output voltage is input to the base of the transistor 30. The transistor 30 is turned on by the output voltage corresponding to a specific temperature. Accordingly, the relay switch 40 is also turned on and operates the buzzer 50 and the light emitting diode 60.
The conventional alarm using the thermistor 10 is widely used. However, as shown in FIG. 1, the conventional alarm has complicated circuitry because it includes the amplifier 20, the transistor 30, and a plurality of resistors.
Another example of the temperature sensor is a temperature sensor using a bimetal. Although the temperature sensor using a bimetal is also widely used, bimetal has a problem in that the range of specific temperatures is too wide.